The invention relates to an electronic proximity switch with a member generating an electromagnetic alternating field, which is excited by an oscillator, an influencing of the alternating field, induced in particular by the approach of a trigger, being evaluated by an electronic signal-evaluation stage, including an amplifier stage, for the emission of an output signal.
A proximity switch of this type is already known for example from DE 40 31 252 C1. The proximity switch, which in that case is an inductive proximity switch, includes an oscillator, which generates an electromagnetic oscillation. This oscillation is coupled out as an electromagnetic alternating field by means of the oscillator coil. Provided in that case are two coils, connected in opposition in order to act as a differential coil arrangement, which are influenced by the electromagnetic field. If the electromagnetic field built up by the transmitting coil is disturbed by the approach of a metallic object, the fields in the two receiver coils alter. The differential signal is fed after amplification to a threshold switch or a rectifier and to a signal amplifier.
A proximity switch which operates on the same principle is described by DE 198 34 071.0.
Similarly, DE 44 29 314.3 describes a proximity switch with a differential coil arrangement.
DE 196 23 969 A1 describes a proximity switch, in particular a capacitive proximity switch, with a sensor electrode, in particular a conducting sensor surface, and an oscillator which applies an alternating voltage to the sensor electrode and the output signal of which is fed in a demodulated form to a threshold switch for the emission of the output signal, the switching distance of the proximity switch being adjustable. In this solution too, it is possible to speak of a field-generating member and a field-influenced member.
The invention is based on the object of reducing the interference susceptibility of an electronic proximity switch of the generic type.
Claim 1 describes as a solution to achieve the object the teaching that the signal to be evaluated has the oscillator frequency applied to it before its evaluation in the signal-evaluation stage. The signal to be evaluated is preferably emitted by a field-influenced member. This development on the one hand allows an offset on the measuring signal to be eliminated. On the other hand, it allows interference signals to be filtered out. In a first variant of the invention, it is provided that the inverted oscillator frequency is added to the measuring signal to compensate for offset. This may take place with the aid of an inverter and an inverting amplifier. As a result, an offset on the measuring signal is subtracted by the feeding in of the oscillator frequency of an identical phase. The amplitude of the oscillator signal fed in is in this case tuned to the uninfluenced amplitude of the measuring signal. Furthermore, it may be provided that at least two members which can be influenced by the field are provided. These members can be influenced by the field in different ways. A differential signal of these two measuring signals is then preferably used for the evaluation. In this case, it may be provided that each of the measuring signal [sic] emitted by the at least two members which can be influenced by the field has the oscillator frequency applied to it. In this case, either the fed-in oscillator frequency can be adapted to the amplitude of the uninfluenced measuring signal, or vice versa. In a preferred configuration, the member generating an electromagnetic field is an electrode, in particular a shielding electrode of a capacitive proximity switch. The members influenced by the field may then be formed by a main electrode or a compensation electrode. The compensation electrode in this case preferably has a lesser remote effect than the main electrode, so that the compensation electrode compensates for interferences in the direct vicinity of the sensor surface. In particular, an arrangement of this type can be used to compensate for voltage drifts and temperature drifts. The measuring signal subjected to addition is preferably passed to a mixer, to which the oscillator signal is likewise applied, the measuring signal being logically multiplied there by the oscillator frequency. A preferred area of use of the invention is that of capacitive proximity switches, in which the main electrode has a circular structure which is surrounded by the compensation electrode in an annular manner. A variant provides that a rotary trigger is located in front of the main electrode, can rotate about the center of the circle of the main electrode and effects a change in capacitance according to the angle of rotation. In this exemplary embodiment, the compensation electrode is preferably eccentrically disposed.
An independent variant of the teaching provided in claim 1 provides that the measuring signal emitted by the field-influenced member [sic] is logically multiplied in a mixer by the oscillator frequency. This has the consequence that substantially only the signals which are both in frequency conformity and phase conformity are evaluated. The mixer preferably has a square-wave signal applied to it, which is formed for example by a threshold switch from the sinusoidal oscillator signal. In this case, the mixing is substantially confined to a sign operation. If the measuring signal is in phase conformity with the oscillator frequency, the two half-waves are rectified by the multiplication. If there is a phase shift, both negative and positive signal contributions are mixed, so that a reduction in level is obtained in the downstream low-pass filter on account of the signal smoothing there. Measuring signals which have a frequency other than the oscillator frequency generate a beat-like mixing output signal, which can likewise be filtered out by the low-pass filter. In a preferred configuration, the proximity switch is in the form of an inductive proximity switch. It is also possible, however, for the circuitry according to the invention to be associated with a capacitive proximity switch. A variant of the invention provides that the measuring signal is formed from a difference between two individual signals. The field-generating member is preferably a transmitting coil. The member influenced by the field may be a single coil or one or more single coils. The member influenced by the field may, inter alia, be connected as a differential coil arrangement. It is also provided, however, that the member influenced by the field is a single coil and, in particular, merely a measuring coil. The member generating the field and the member which can be influenced by the field may, in addition, be capacitor-plate-like electrodes. In this case, the proximity switch in question is a capacitive proximity switch. In a further variant, it is provided that the oscillator frequency is modulated. In this variant, for example, the transmitting coil is not operated with a fixed frequency but with a modulated frequency. This modulated frequency is also passed to the mixer. The modulation of the oscillator signal preferably takes place in a randomly controlled manner. For the random modulation, a noise generator may be provided in particular. This configuration allows a plurality of proximity switches to be disposed close together, without the alternating fields of the individual proximity switches disturbing one another. Each proximity switch can in this case also oscillate with an individual fixed frequency. The other fixed frequencies are then mixed out as a consequence of the mixer stage. If the oscillator is modulated by a noise generator, it is statistically extremely improbable for the same frequencies to be transmitted from neighboring proximity switches over significant periods, so that this variant also leads to improve suitability for packet assembly. The circuit may be formed by individual components, for example by a plurality of inverting or non-inverting amplifier stages. It is also provided, however, for the entire mixer circuit, including the threshold switch and amplifier, to be integrated. In a further variant of the invention, it is provided that an oscillating circuit, comprising a coil and a capacitor, is set in oscillation by having an alternating voltage generated by an oscillator applied to it. This oscillating circuit can be damped by the approach of an oscillator, so that the oscillation is interrupted. The alternating voltage present at the oscillating circuit is picked off by means of a coupling capacitor or the like and fed to a multiplier, in particular after amplification. In this multiplier, the alternating voltage picked off at the oscillating circuit is subjected to multiplication by an alternating voltage signal coming directly from the oscillator in such a way that only in-phase signals are fed to the evaluation stage. This has the consequence that extraneous pulses which are received by the oscillating circuit, and in particular voltage peaks, are filtered out in the mixer. This solution can dispense with an additional xe2x80x9creceiving coilxe2x80x9d. On the other hand, here, too, interference suppression can be achieved by the measuring signal having the undisturbed oscillator signal applied to it.
It is regarded as advantageous that the circuit has a symmetrical construction.